oracular (3) Log::Log4perl::Appender::Synchronized.3pm.gz
NAME
Log::Log4perl::Appender::Synchronized - Synchronizing other appenders
SYNOPSIS
use Log::Log4perl qw(:easy);
my $conf = qq(
log4perl.category = WARN, Syncer
# File appender (unsynchronized)
log4perl.appender.Logfile = Log::Log4perl::Appender::File
log4perl.appender.Logfile.autoflush = 1
log4perl.appender.Logfile.filename = test.log
log4perl.appender.Logfile.mode = truncate
log4perl.appender.Logfile.layout = SimpleLayout
# Synchronizing appender, using the file appender above
log4perl.appender.Syncer = Log::Log4perl::Appender::Synchronized
log4perl.appender.Syncer.appender = Logfile
);
Log::Log4perl->init(\$conf);
WARN("This message is guaranteed to be complete.");
DESCRIPTION
If multiple processes are using the same "Log::Log4perl" appender without synchronization, overwrites
might happen. A typical scenario for this would be a process spawning children, each of which inherits
the parent's Log::Log4perl configuration.
In most cases, you won't need an external synchronisation tool like Log::Log4perl::Appender::Synchronized
at all. Log4perl's file appender, Log::Log4perl::Appender::File, for example, provides the "syswrite"
mechanism for making sure that even long log lines won't interleave. Short log lines won't interleave
anyway, because the operating system makes sure the line gets written before a task switch occurs.
In cases where you need additional synchronization, however, you can use
"Log::Log4perl::Appender::Synchronized" as a gateway between your loggers and your appenders. An appender
itself, "Log::Log4perl::Appender::Synchronized" just takes two additional arguments:
"appender"
Specifies the name of the appender it synchronizes access to. The appender specified must be defined
somewhere in the configuration file, not necessarily before the definition of
"Log::Log4perl::Appender::Synchronized".
"key"
This optional argument specifies the key for the semaphore that
"Log::Log4perl::Appender::Synchronized" uses internally to ensure atomic operations. It defaults to
"_l4p". If you define more than one "Log::Log4perl::Appender::Synchronized" appender, it is important
to specify different keys for them, as otherwise every new "Log::Log4perl::Appender::Synchronized"
appender will nuke previously defined semaphores. The maximum key length is four characters, longer
keys will be truncated to 4 characters -- "mylongkey1" and "mylongkey2" are interpreted to be the
same: "mylo" (thanks to David Viner <dviner@yahoo-inc.com> for pointing this out).
"Log::Log4perl::Appender::Synchronized" uses Log::Log4perl::Util::Semaphore internally to perform locking
with semaphores provided by the operating system used.
Performance tips
The "Log::Log4perl::Appender::Synchronized" serializes access to a protected resource globally, slowing
down actions otherwise performed in parallel.
Unless specified otherwise, all instances of "Log::Log4perl::Appender::Synchronized" objects in the
system will use the same global IPC key "_l4p".
To control access to different appender instances, it often makes sense to define different keys for
different synchronizing appenders. In this way, Log::Log4perl serializes access to each appender instance
separately:
log4perl.category = WARN, Syncer1, Syncer2
# File appender 1 (unsynchronized)
log4perl.appender.Logfile1 = Log::Log4perl::Appender::File
log4perl.appender.Logfile1.filename = test1.log
log4perl.appender.Logfile1.layout = SimpleLayout
# File appender 2 (unsynchronized)
log4perl.appender.Logfile2 = Log::Log4perl::Appender::File
log4perl.appender.Logfile2.filename = test2.log
log4perl.appender.Logfile2.layout = SimpleLayout
# Synchronizing appender, using the file appender above
log4perl.appender.Syncer1 = Log::Log4perl::Appender::Synchronized
log4perl.appender.Syncer1.appender = Logfile1
log4perl.appender.Syncer1.key = l4p1
# Synchronizing appender, using the file appender above
log4perl.appender.Syncer2 = Log::Log4perl::Appender::Synchronized
log4perl.appender.Syncer2.appender = Logfile2
log4perl.appender.Syncer2.key = l4p2
Without the ".key = l4p1" and ".key = l4p2" lines, both Synchronized appenders would be using the default
"_l4p" key, causing unnecessary serialization of output written to different files.
Advanced configuration
To configure the underlying Log::Log4perl::Util::Semaphore module in a different way than with the
default settings provided by Log::Log4perl::Appender::Synchronized, use the following parameters:
log4perl.appender.Syncer1.destroy = 1
log4perl.appender.Syncer1.mode = sub { 0775 }
log4perl.appender.Syncer1.uid = hugo
log4perl.appender.Syncer1.gid = 100
Valid options are "destroy" (Remove the semaphore on exit), "mode" (permissions on the semaphore), "uid"
(uid or user name the semaphore is owned by), and "gid" (group id the semaphore is owned by),
Note that "mode" is usually given in octal and therefore needs to be specified as a perl sub {}, unless
you want to calculate what 0755 means in decimal.
Changing ownership or group settings for a semaphore will obviously only work if the current user ID owns
the semaphore already or if the current user is "root". The "destroy" option causes the current process
to destroy the semaphore on exit. Spawned children of the process won't inherit this behavior.
Semaphore user and group IDs with mod_perl
Setting user and group IDs is especially important when the Synchronized appender is used with mod_perl.
If Log4perl gets initialized by a startup handler, which runs as root, and not as the user who will later
use the semaphore, the settings for uid, gid, and mode can help establish matching semaphore ownership
and access rights.
DEVELOPMENT NOTES
"Log::Log4perl::Appender::Synchronized" is a composite appender. Unlike other appenders, it doesn't log
any messages, it just passes them on to its attached sub-appender. For this reason, it doesn't need a
layout (contrary to regular appenders). If it defines none, messages are passed on unaltered.
Custom filters are also applied to the composite appender only. They are not applied to the sub-
appender. Same applies to appender thresholds. This behaviour might change in the future.
LICENSE
Copyright 2002-2013 by Mike Schilli <m@perlmeister.com> and Kevin Goess <cpan@goess.org>. This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
AUTHOR
Please contribute patches to the project on Github:
http://github.com/mschilli/log4perl
Send bug reports or requests for enhancements to the authors via our
MAILING LIST (questions, bug reports, suggestions/patches): log4perl-devel@lists.sourceforge.net
Authors (please contact them via the list above, not directly): Mike Schilli <m@perlmeister.com>, Kevin
Goess <cpan@goess.org>
Contributors (in alphabetical order): Ateeq Altaf, Cory Bennett, Jens Berthold, Jeremy Bopp, Hutton
Davidson, Chris R. Donnelly, Matisse Enzer, Hugh Esco, Anthony Foiani, James FitzGibbon, Carl Franks,
Dennis Gregorovic, Andy Grundman, Paul Harrington, Alexander Hartmaier David Hull, Robert Jacobson,
Jason Kohles, Jeff Macdonald, Markus Peter, Brett Rann, Peter Rabbitson, Erik Selberg, Aaron Straup Cope,
Lars Thegler, David Viner, Mac Yang.